Process for reducing the amount of calcium contained in magnesium-base waste sulfite liquor by the use of an ammonium compound and additional calcium

ABSTRACT

The amount of calcium contained in magnesium-base waste sulfite liquor is reduced by precipitating the calcium as calcium sulfite and separating the precipitate from the remaining waste sulfite liquor solution. The precipitation is caused by dissolving in the waste sulfite liquor (1) additional calcium, i.e., beyond the amount of calcium initially contained in the waste sulfite liquor; (2) an ammonium compound selected from the group consisting of anhydrous ammonia, ammonium sulfite, aqueous ammonia, ammonium carbonate, ammonium magnesium carbonate and ammonium hydroxide; and (3) a source of sulfite ions.

United States Patent 1 Lahtvee et al.

[ PROCESS FOR REDUCING THE AMOUNT OF CALCIUM CONTAINED IN MAGNESIUM-BASEWASTE SULFITE LIQUOR BY THE USE OF AN AMMONIUM COMPOUND AND ADDITIONALCALCIUM [75] Inventors: Toivo Lahtvee, Scarborough,

Ontario; Bal Krishan Sethi, Toronto, Ontario, both of Canada; WilliamHubbard Stark, Cincinnati, Ohio [73] Assignee: Spring Chemicals Limited,Toronto,

Ontario, Canada 22 Filed: Sept. 22, 1971 [21] Appl. No.: 182,832

[52 us. Cl 4i3/512, 423 1 9, 4231158 [51] Int. Cl....C0lb 17/45, COlb17/46, C011) 17/62, COlb 17/98 [58] Field of Search 423/512-520,544-562,

[56] References Cited.

UNITED STATES PATENTS 2,644,748 7/1953 Cunningham 423/512 X 12/1947 Kahnet al. 423/558 X Primary Examiner-Earl C. Thomas AssistantExaminer-Gregory A. Heller Attorney, Agent, or Firm-Burns, Doane,Swecker &

Mathis [57] ABSTRACT The amount of calcium contained in magnesium-basewastev sulfite liquor is reduced by precipitating the calcium as calciumsulfite and separating the precipitate from the remaining waste sulfiteliquor solution. The precipitation is caused by dissolving in the wastesulfite liquor (1) additional calcium, i.e., beyond the amount ofcalcium initially contained in the waste sulfite liquor; (2) an ammoniumcompound selected from the group consisting of anhydrous ammonia,ammoniurn sulfite, aqueous ammonia, ammonium carbonate, ammoniummagnesium carbonate and ammonium hydroxide; and (3) a source of sulfiteions.

' 12 Claims, N0 Drawings Trubey et a1. 423/561 X y PROCESS FOR REDUCINGTHE AMOUNT OF CALCIUM CONTAINED IN MAGNESIUM-BASE WASTE SULFITE LIQUORBY THE USE OF AN AMMONIUM COMPOUND AND ADDITIONA CALCIUM This inventionrelates to a process fortreating magnesium-base waste sulfite liquor.More particularly, the present invention relates to a process forprecipitating calcium from magnesium-base waste sulfite liquor ascalcium sulfite.

BACKGROUND OF THE INVENTION Sulfite pulping processes are a group ofcommonly used processes for obtaining pulp from wood or other cellulosicmaterials. In the sulfite processes, the cellulosic materials arenormally treated with an aqueous solution containing an alkali metalbisulfite, alkaline earth metal bisulfite, or ammonium bisulfite. Insome instances, when less sulfur dioxide is present in the aqueoussolutions, all or portions of the metal or the ammonium cations may bepresent in the monosulfite form. In still other instances, excess sulfurdioxide may be present in the solutions, over and above the amountnecessary to maintain the cations in the bisulfite form,

thus giving rise to what is specifically referred to as an acid sulfitesolution. All of these solutions are usually collectively or genericallyreferred to as sulfite cooking liquors.

The sulfite cooking liquors dissolve much of the lignin, some of thehemi-cellulose, as well as other components of the cellulosic materialemployed, leaving the majority of the cellulose fibers undissolved. Theundissolved cellulose fibers are separated from the solution and theremaining solution containing the dissolved components is known as wastesulfite liquor. The waste sulfite liquor has dissolved thereinlignosulfonates and other organic and inorganic compounds of the cationused in the cooking liquor, and of any other cations that may have beenpresent in the sulfite cooking liquor or the cellulosic material.

One of the commonly used sulfite cooking liquors contains magnesium asthe principal cation, and accordingly, the resulting waste sulfiteliquor is known as magnesium-base waste sulfite liquor.

The magnesium-base waste sulfite liquor solutions contain valuablechemicals such as organic and inorganic salts of magnesium, sulfur invarious forms, and a number of organic constituents. As a result,efforts have been made to develop methods for recovering these valuablecomponents, and particularly for recovering sulfur dioxide and magnesiumfor use in the prep aration of fresh sulfite cooking liquor and forrecovering heating values from the organics.

Some of the recovery processes suggested heretofore have includedsubjecting the magnesium-base waste sulfite liquor solutions toevaporation and burning operations to obtain an ash containing magnesiumoxide. Unfortunately, because of the amounts of calcium compoundspresent in the waste liquor, serious scaling problems occur in theevaporators. The recovered ash is also contaminated by the calciumcompounds.

The calcium present in magnesium-base waste sulfite liquor comes fromthe wood used in the pulping process, from impurities in the water usedto make the sulfite cooking solution, and from the various chemicalsused in the cooking process. The calcium compounds recover the magnesiumand other valuable components.

In order to significantly reduce the calcium content, however, it isnormally necessary to use large amounts of ammonium hydroxide andsulfite ions. A process has now been found whereby less ammoniumhydroxide and sulfite ions can be employed to obtain significantreductions in the amount of calcium present.

*OBJECTS AND SUMMARY OF THE INVENTION The primary object of the presentinvention is to provide a process for reducing the amount of calciumcontainedin magnesium-base waste sulfite liquor.

A further object is to provide a process for reducing the calciumcontent of magnesium-base waste sulfite liquor which is more economicaland effective than those processes used heretofore.

Other objects of the present invention will be apparent to those skilledin the art from the following more detailed description.

According to the present invention, a source of sulfite ions, additionalcalcium (that is, calcium in excess of the amount present in themagnesium-base waste sulfite liquor obtained from the sulfite pulpingprocess) and at least one ammonium compound selected from the classconsisting of anhydrous ammonia, ammonium sulfite, aqueous ammonia,ammonium carbonate, ammonium. magnesium carbonate and ammonium hydroxideare all dissolved in a magnesium-base waste sulfite liquor. Thesematerials are added to the magnesium-base waste sulfite liquor toprecipitate the calcium therefrom as calcium sulfite. I

The essence of the present invention is the surprising discovery that byadding calcium to the magnesiumbase waste sulfite liquor, any givenlevel of calcium reduction can be obtained using less sulfite ions andless ammonium compound than when no added calcium is employed.

In its broadest aspect, the present invention is intended to coverdissolving any additional amount of calcium in the magnesium-base wastesulfite liquor in excess of the amount ofcalcium contained in the wasteliquor recovered from the sulfite pulping process. In one of itspreferred aspects, calcium is added in an amount sufficient to yield awaste sulfite liquor containing up to about 1000 ppm of dissolvedcalcium prior to precipitation of calcium sulfite.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The termmagnesium-base waste sulfite liquor as 'used herein is intended toinclude all waste'sulfite weight basis. it is to be understood that theoriginal sulfite cooking liquor can be of any specific type, rangingfrom neutral to the acid sulfite form depending on the amount of sulfurdioxide dissolved therein. The only requirement is that magnesium be themajor or principal cation. Normally, magnesium-base waste sulfiteliquors contain between about 2,000 and 6,500 ppm of magnesium, betweenabout 250 and 600 ppm of calcium and below about 100 ppm of othercations such as strontium, barium, iron, aluminum, sodium, potassium andthe like. The pH of the magnesium-base waste sulfite liquor as obtainedfrom the pulping processes is normally between about 1.5 and 4.5.

The calcium in magnesium-base waste sulfite liquor is normally dissolvedin the liquor in the form of various organic and inorganic salts such aslignosulfonates, oxalates, acetates and sulfates as well as in the formof calcium bisulfite.

By the use of the process of the present invention the calcium contentof magnesium-base waste sulfite liquor can be reduced. If desired, thecalcium level can be reduced to below about 100 ppm and even to about 50ppm or lower (based on the weight of the waste sulfite liquor solution).When the calcium level is below about 100 ppm, the magnesium-base wasteliquor can be more effectively and economically processed to recovermagnesium and other valuable components employing such techniques asevaporators, recovery boilers, absorbers and the like. This is becausethe scaling, if. any, which occurs at such low calcium levels is noteconomically prohibitive. An additional advantage in obtaining lowcalcium levels is that the scaling problem which occurs in variouspulping process equipment will be minimized, if not eliminated, forexample, when magnesium cation is recovered for use in making freshcooking liquor.

Most importantly, however, is that by the process of the presentinvention less ammonium compound and sulfite ions are needed as comparedto the prior art processes.

As previously mentioned, the ammonium compound isselected from the classconsisting of anhydrous ammonia, ammonium sulfite, aqueous ammonia,ammonium carbonate, ammonium magnesium carbonate and ammonium hydroxide.These ammonium compounds are soluble in magnesium-base waste sulfiteliquor or can be made soluble therein by increasing the sulfite ionconcentration of the liquor. Mixtures of the ammonium compounds may beemployed, for example aqueous ammonia and ammonium sulfite. Preferably,ammonium hydroxide, aqueous ammonia or ammonium sulfite is used.

In addition to the ammonium compound, additional" calcium is alsodissolved in the magnesium-base waste sulfite liquor, that is, in excessof the original amount of calcium present in the magnesium-base wastesulfite liquor obtained from the pulping process.

The source of additional calcium may be any .compound or material whichwill dissolve (or be dissolved by the presence of sulfite ions) in themagnesium-base waste sulfite liquor to provide dissolved calcium, i.e.,calcium ions, therein. The calcium compound or material used should notcause adverse side reactions, interfere with the precipitation ofcalcium sulfite and its recovery, or add any deleterious substance tothe waste sulfite liquor. Several calcium compounds that may be usedinclude calcium hydroxide, calcium oxide, calcium carbonate, calciumsulfite, and minerals containing calcium such as dolomite, brucite,limestone and chalk.

Any additional amount of calcium may be dissolved in the magnesium-basewaste sulfite liquor in accordance with the present invention. Aspreviously mentioned, the additional calcium will enable the desiredcalcium reduction to take place with the use of less ammonium compoundand less sulfite ions. In other words, if it is desired to reduce thecalcium content of magnesium-base waste sulfite liquor from 300 ppm, forexample, to ppm, less ammonium compound and less sulfite ion will beused when additional calcium is added as compared to when no additionalcalcium is added.

While any amount of additional calcium may be dissolved in the wastesulfite liquor in accordance with the present invention, preferably, theadditional calcium is dissolved in amounts such that the waste sulfiteliquor contains up to about 1,000 ppm of calcium dissolved therein,based on the weight of the waste sulfite liquor solution (this includesthe calcium already dissolved in the original waste liquor obtained fromthe pulping process). It has been'found that little additionalimprovement is obtained when more than about 1,000 ppm of dissolvedcalcium are contained in the waste sulfite liquor. In other words,increasing the calcium content from between about 250 and 600 ppm (theusual amount of calcium dissolved in magnesium-base waste sulfiteliquor) to about 1,00 1,000 ppm is all that is usually necessary inaccordance with the present invention. However, higher amounts ofdissolved calcium can be used if so desired.

Moreover, each incremental addition of dissolved calcium will normallygive a further improvement. That is, raising the dissolved calcium levelto 700 ppm will usually require less ammonium compound and sulfite ionsto obtain a certain calcium reduction, as compared to raising thedissolved calcium level to only 500 ppm. A further raise in thedissolved calcium level to 900 ppm will usually require still loweramounts of the other ingredients, and so on, up to about 1,000 ppm.

A particularly preferred method for reducing the calcium content ofmagnesium-base waste sulfite liquor includes dissolving one or more ofthe above calcium compounds or materials in the liquor to obtain up toabout and preferably about 1,000 ppm of calcium; and dissolving one ormore of the above ammonium compounds in the liquor to obtain above about7,000 ppm and preferably between about 7,000 and 13,000 ppm ofammonia-magnesium cations" dissolved in the liquor.

The expression ammonia-magnesium cations as used herein means the amountof magnesium and the amount of ammonia, expressed as magnesiumequivalents, which are dissolved in the waste sulfite liquor. The ppm ofdissolved ammonia are converted to the ppm of dissolved magnesiumequivalent by multiplying, the ppm of ammonia by a factor of 0.715. Theppm of magnesium and the ppm magnesium equivalent of ammonia are thenadded to give ammoniamagnesium cations which are dissolved in the wastesulfite liquor.

If more than about 50 ppm of sodium and/or potassium are dissolved inthe magnesium-base waste sulfite liquor, the respective amounts ofsodium and potassium should also be converted to magnesium cquivalentsand included in the ammonia-magnesium cation" total. The ppm ofdissolved sodium are multiplied by 0.529 and the ppm of dissolvedpotassium by 0.31 l to convert each to ppm of dissolved magnesiumequivalents.

It is important to note that the ppm of dissolved magnesium, ammonia,sodium and/or potassium are to include not only the amounts that may beadded during the calcium removal treatment, but also the amounts presentin the original magnesium-base waste sulfite liquor obtained from thepulping process. The above amounts of dissolved calcium andammonia-magnesium cations should be obtained in the waste sulfite liquorsolution before the calcium sulfite precipitation is complete. Forexample, some of the calcium and ammonium compound may be added ascalcium sulfite is precipitating, provided the total desired amount isdissolved in the waste sulfite liquor before completing precipitation.With respect to the calcium, it is preferred that all of the additionalcalcium which is to be added be dissolved in the waste sulfite liquorbefore any calcium sulfite precipitation occurs, for example, prior toraising the pH of the waste sulfite liquor solution above about 4.0.

In addition to the added calcium and the ammonium compound, themagnesium-base waste sulfite liquor is also provided with a source ofsulfite ions. The presence of sulfite ions in the waste sulfite liquorserves several functions. First, sufficient sulfite ions must beavailable to dissolve the ammonium and calcium compounds or materialsthat are added to the waste sulfite liquor. in addition, sulfite ionsmust be provided to combine with calcium, and, under thereactionconditions used, to cause calcium to precipitate as calcium sulfite.Furthermore, the sulfite ions assist in adjusting the pH of the wastesulfite liquor, for example, by keeping the pH fromv rising above thedesired level as the ammonium and calcium components are added.

.Any convenient source of sulfite ions may be used in the presentinvention such as sulfur dioxide, magnesium sulfite, ammonium sulfite,calcium sulfite and sodium sulfite. Preferably, sulfur dioxide orammonium sulfite is used. In the instances when it is necessary toprovide sulfite ions for purposes of dissolving calcium and/or magnesiumcompounds or materials, the sulfite ion is normally sulfur dioxide.

The sulfite ions are usually provided in sufficient amount to adjust thepH so that at the completion of the dissolution of the added calcium andammonium compound, the pH of the waste sulfite liquor is between about 5and 8, preferably between about 5.5 and 6.5, and more particularly atabout 5.7. Under these conditions, calcium contained in the wastesulfite liquor precipitates as calcium sulfite.

The calcium sulfite precipitate is separated from the remaining wastesulfite liquor solution by any conventional technique such asfiltration, settling and the like.

The additional calcium, ammonium compound and sulfite ions may be addedto the magnesium-base waste sulfite liquor in any convenient mannerknown to those skilled in the art and in any order desired. Onepreferred sequence, however, is to maintain the pH of the waste sulfiteliquor below about 4 (such as by the addition of sulfur dioxide) untilall of the additional calcium to be added is dissolved and a majorportion of the ammonium compound to be added is dissolved, and then addthe remaining ammonium compound along with more sulfur dioxide, ifneeded, to reach the desired I final pH.

The addition of the above substances to the waste sulfite liquor as wellas the precipitation and separation of calcium sulfite precipitate fromthe remaining solution is conveniently conducted at atmospheric pressureand at a temperature of above about 50C., such as between about 60 and100C., and preferably between about and C. If desired, higher pressuresand temperatures can be used provided adverse reactions are avoided.

Once the ammonium compound and added calcium are dissolved in the wastesulfite liquor solution and the final desired pH is obtained, up toabout 30 minutes or more of residence time is usually sufficient toobtain completion of calcium sulfite precipitation.

After the calcium sulfite precipitate is removed from the remainingwaste sulfite liquor solution it may be treated to obtain calciumcompounds and sulfur dioxide. The waste solution remaining, i.e.,filtrate, can be treated to obtain magnesium compounds as well as othervaluable components. This may be accomplished,

The following examples illustrate methods of carry-.

ing out the present invention.

EXAMPLES The following Table I contains a listing of 31 experiments,denoted as Run Nos. 1 to 31, which were conducted to demonstrate theresults that can be obtained by the process of the present invention.

Magnesium-base waste sulfite liquor was obtained from a pulping processand was analyzed to determine its calcium and magnesium contents. 4

Each of the magnesium-base waste sulfite liquors used in the Runs 1 to31 were found to contain 300 ppm of calcium, all of which was dissolvedin the liquor. However, the liquors, as obtained from the pulpingprocess, contained varying amounts of magnesium, as can be seen inColumn 3 of the Table l.

The pH of the liquors as received from thepulping mills was about 3.

in Runs 1 to 5 no additional calcium was added to the liquor. However,in each of the remaining runs various amounts of calcium hydroxide wereadded to the liquors. For example, in Runs 6 to 10, it can be seen fromColumn 2 of the Table that 350 ppm of calcium were'dissolved in thewaste sulfite liquor feed to obtain a total of 650 ppm. As indicated inColumn 2, the additional calcium was dissolved prior to any calciumsulfite precipitation. I

In each of the Runs 1 to 31 ammonium hydroxide was also added to thewaste sulfite liquor feed in order to dissolve ammonium ions in theliquor. The amount of ammonia added in each run is shown in Column 5,and the magnesium equivalent is in parenthesis. The

original magnesium-base waste sulfite liquors did not contain anymeasurable amounts of ammonia.

Sulfur dioxide was used in each run to assist in dissolving the calciumhydroxide and ammonium hydroxin Run 5. Also compare Runs 4 and 8; 4 and12', and 4 and 22. The few variations in the above data such as Run 10are within the range of experimental error. Similar results are obtainedwhen other ammonium comammonia-magnesium cations were required to reducethe calcium level from 300 ppm to 46 ppm, while in the latter, when 640ppm of calcium were added, only 9,210 ppm of ammonia-magnesium cationswere needed. About 5,500 more ppm of ammonia was added ide in theliquor, and to adjust the pH of the liquor to 5 pounds such as ammoniumsulfite and anhydrous ama final value of 5.7. In each run, the liquorwas held at monia are used, the final pH of 5.7 for 20 minutes, withcontinuous stir- The principles, preferred embodiments and modes ofring. The temperature of the liquor was held constant operation of thepresent invention have been described at 70C. in the foregoingspecification. The invention which is At the end of the 20 minute periodcalcium sulfite l intended to be protected herein, however, is not to beprecipitation had been completed. A sample of the soconstrued as limitedto the particular forms disclosed, lution was taken, the precipitateseparated therefrom since these are to be regarded as illustrativerather than by filtration and the filtrate analyzed. restrictive.Variations and changes may be made by C l 4 of T bl I li t h amount fmagnesium those skilled in the art without departing from the spiritwhich was dissolved in the filtrate, Column the ppm 15 0f the pinvention of ammonia (same as was added) and Column 7 the We Claimi m fcal ium, 1. A process for treating magnesium-base waste sul- Column 6 ofTable I is the ammonia-magnesium cat fite liquor containing calcium inorder to l'ClUC6 the ions which are the total of Column 4 and themagneamount of calcium contained therein by precipitating Sin e uivalenfigure f C l 5, 20 calcium from the waste sulfite liquor as calciumsulfite,

TABLE 1 Cu (1 p Mg (pp Dissolved in WSL in WSL Obtained NH (ppm) AddedAmmonia- Run Prior to Precipifrom Pulping Mg (ppm) and Mg EquivalentMagnesium Ca (ppm) No. tution Process in Filtrate (ppm) Cations inFiltrate The filtrate was separated from the precipitate after 30minutes of reaction time.

As can be seen from the above Table, when calcium which processcomprises: is added to the magnesium-base waste sulfite liquor a.dissolving an ammonium compound in said waste lower amounts of ammoniaare required. Compare, for sulfit liquor in an amount Sufficient toobtain 8. example, 'Runs 5 and 13. In the former, 13,150 ppm wastesulfite liquor containing above about 7,000

ppm of ammonia-magnesium cations dissolved therein prior to thecompletion of precipitation of said calcium sulfite,'said ammoniumcompound being selected from the class consisting of anhydrous ammonia,ammonium sulfite, aqueous ammonia, ammonium carbonate, ammoniummagnesium carbonate, and ammonium hydroxide;

b. increasing the calcium ion concentration in said waste sulfite liquorso as to obtain a waste sulfite liquor solution containing up to about1,000 ppm of calcium dissolved therein prior to the completion ofprecipitation of said calcium sulfite;

c. adding a source of sulfite ions to said waste sulfite liquor in atleast an amount sufficient to cause the calcium contained therein toprecipitate as calcium sulfite and to maintain the pH of the wastesulfite liquor solution between about and 8 during the calcium sulfiteprecipitation; and

d. separating said precipitated calcium sulfite from the remaining wastesulfite liquor solution.

- 2. The process of claim 1 wherein said additional calcium ions areadded to the waste sulfite liquor by dissolving in said waste sulfiteliquor a calcium compound selected from the class consisting of calciumhydroxide, calcium oxide, calcium carbonate, calcium sulfite, brucite,limestone, chalk and dolomite.

3. The process of claim 2 wherein the magnesiumbase waste sulfite liquorto be treated contains between about 250 and 600 ppm of calcium, andwherein said calcium compound is dissolved in said waste sulfite liquorto obtain about 1,000 ppm of calcium dissolved therein prior to theprecipitation of calcium sulfite.

4. A process for treating magnesium-base waste sulfite liquor containingcalcium in order to reduce the amount of calcium contained therein byprecipitating calcium from the waste sulfite liquor as calcium sulfite,which process comprises:

a.' dissolving an ammonium compound in said waste sulfite liquor in anamount sufficient to obtain a waste sulfite liquor containing aboveabout 7,000 ppm of ammonia-magnesium cations dissolved therein prior tothe completion of precipitation of said calcium sulfite, said ammoniumcompound being selected from the class consisting of anhydrous ammonia,ammonium sulfite, aqueous ammonia, ammonium carbonate, ammoniummagnetaining up to about 1,000 ppm of calcium dissolved therein prior tothe completion of precipitation of said calcium sulfite;

c. adding to said waste sulfite liquor a member selected from the classconsisting of sulfur dioxide, magnesium sulfite, ammonium sulfite,calcium sulfite and sodium sulfite so as to provide sulfite ions in saidwaste sulfite liquor in at least an amount sufficient to cause thecalcium contained therein to precipitate as calcium sulfite and tomaintain the pH of the waste sulfite liquor solution between about 5 and8 during the calcium sulfite precipitation; and

d. separating said precipitated calcium sulfite from the remaining wastesulfite liquor solution.

5. The process of claim 4 wherein the magnesiumbase waste sulfite liquorto be treated contains between about 250 and 600 ppm of calcium, andwherein said calcium compound is dissolved in said waste sulfite liquorto obtain about 1,000 ppm of calcium dissolved therein prior to theprecipitation of calcium sulfite.

6. The process of claim 4 wherein said ammonium compound is dissolved insaid waste sulfite liquor in an amount sufficient to obtain a wastesulfite liquor containing between about 7,000 and 13,000 ppm ofammonia-magnesium cations dissolved therein prior to the completion ofprecipitation of said calcium sulfite.

7. The process of claim 6 wherein the magnesiumbase waste sulfite liquorto be treated contains between about 250 and 600 ppm of calcium, and.wherein said calcium compound is dissolved in said waste sulfite liquorto obtain about 1,000 ppm of calcium dissolved therein prior to theprecipitation of calcium sulfite.

8. The process of claim 7 wherein sulfur dioxide is added to provide thesulfite ions in said waste sulfite liquor.

9. The process of claim 7 wherein the pH of the waste sulfite liquorsolution is between about 5.5 and 6.5 during the calcium sulfiteprecipitation.

10. The process of claim 9 wherein the calcium compound is calciumhydroxide and the sulfite ions are provided by adding sulfur dioxide.

11. The process of claim 10 wherein the ammonium compound is ammoniumsulfite.

12. The process of claim 10 wherein the ammonium compound is ammoniumhydroxide.

2. The process of claim 1 wherein said additional calcium ions are addedto the waste sulfite liquor by dissolving in said waste sulfite liquor acalcium compound sElected from the class consisting of calciumhydroxide, calcium oxide, calcium carbonate, calcium sulfite, brucite,limestone, chalk and dolomite.
 3. The process of claim 2 wherein themagnesium-base waste sulfite liquor to be treated contains between about250 and 600 ppm of calcium, and wherein said calcium compound isdissolved in said waste sulfite liquor to obtain about 1,000 ppm ofcalcium dissolved therein prior to the precipitation of calcium sulfite.4. A process for treating magnesium-base waste sulfite liquor containingcalcium in order to reduce the amount of calcium contained therein byprecipitating calcium from the waste sulfite liquor as calcium sulfite,which process comprises: a. dissolving an ammonium compound in saidwaste sulfite liquor in an amount sufficient to obtain a waste sulfiteliquor containing above about 7,000 ppm of ammonia-magnesium cationsdissolved therein prior to the completion of precipitation of saidcalcium sulfite, said ammonium compound being selected from the classconsisting of anhydrous ammonia, ammonium sulfite, aqueous ammonia,ammonium carbonate, ammonium magnesium carbonate, and ammoniumhydroxide; b. adding to said waste sulfite liquor a calcium compoundselected from the class consisting of calcium hydroxide, calcium oxide,calcium carbonate, calcium sulfite, brucite, limestone, chalk anddolomite so as to obtain a waste sulfite liquor solution containing upto about 1,000 ppm of calcium dissolved therein prior to the completionof precipitation of said calcium sulfite; c. adding to said wastesulfite liquor a member selected from the class consisting of sulfurdioxide, magnesium sulfite, ammonium sulfite, calcium sulfite and sodiumsulfite so as to provide sulfite ions in said waste sulfite liquor in atleast an amount sufficient to cause the calcium contained therein toprecipitate as calcium sulfite and to maintain the pH of the wastesulfite liquor solution between about 5 and 8 during the calcium sulfiteprecipitation; and d. separating said precipitated calcium sulfite fromthe remaining waste sulfite liquor solution.
 5. The process of claim 4wherein the magnesium-base waste sulfite liquor to be treated containsbetween about 250 and 600 ppm of calcium, and wherein said calciumcompound is dissolved in said waste sulfite liquor to obtain about 1,000ppm of calcium dissolved therein prior to the precipitation of calciumsulfite.
 6. The process of claim 4 wherein said ammonium compound isdissolved in said waste sulfite liquor in an amount sufficient to obtaina waste sulfite liquor containing between about 7,000 and 13,000 ppm ofammonia-magnesium cations dissolved therein prior to the completion ofprecipitation of said calcium sulfite.
 7. The process of claim 6 whereinthe magnesium-base waste sulfite liquor to be treated contains betweenabout 250 and 600 ppm of calcium, and wherein said calcium compound isdissolved in said waste sulfite liquor to obtain about 1,000 ppm ofcalcium dissolved therein prior to the precipitation of calcium sulfite.8. The process of claim 7 wherein sulfur dioxide is added to provide thesulfite ions in said waste sulfite liquor.
 9. The process of claim 7wherein the pH of the waste sulfite liquor solution is between about 5.5and 6.5 during the calcium sulfite precipitation.
 10. The process ofclaim 9 wherein the calcium compound is calcium hydroxide and thesulfite ions are provided by adding sulfur dioxide.
 11. The process ofclaim 10 wherein the ammonium compound is ammonium sulfite.
 12. Theprocess of claim 10 wherein the ammonium compound is ammonium hydroxide.